Steel used in ships, marine structures, pressure vessels, and the like is welded to form structures with desired shapes. Therefore, from the perspective of structural safety, these steels are not only required to have base metal with high strength and excellent toughness but also to have excellent toughness in weld joints (weld metal and Heat-Affected Zone (HAZ)).
The absorbed energy by a Charpy impact test has mainly been used as the basis for evaluating the toughness of steel. In recent years, however, a Crack Tip Opening Displacement test (CTOD test; the evaluation results of this test are referred to below as CTOD property or CTOD value) has often been used for greater reliability. This test evaluates the resistance to occurrence of brittle fracture by generating a fatigue precrack in a test piece at the location of toughness evaluation, subjecting the test piece to three-point bending, and measuring the amount of the crack opening (plastic deformation volume) immediately before fracture.
Since a fatigue precrack is used in this CTOD test, an extremely small region is evaluated for toughness. If a local brittle zone exists, a low toughness may in some cases be indicated, even if a good toughness is obtained with a Charpy impact test.
When forming a multilayer fill weld in a thick steel plate or the like, the local brittle zones easily occur in the Heat-Affected Zone (HAZ), which is subjected to a complicated thermal history. Specifically, the bond (the boundary between weld metal and base metal) and a region in which the bond is formed into a dual phase region by reheating (a region in which coarse grains are formed in the first cycle of welding and which is heated into a ferrite and austenite dual phase region by the subsequent welding pass, hereinafter referred to as a dual phase reheating area) become local brittle zones.
Since the bond is exposed to a high temperature just below the melting point, austenite grains are coarsened and are likely to be transformed, by the subsequent cooling, into an upper bainite structure that has a low toughness. Therefore, the toughness of the matrix itself is low. Furthermore, brittle structures such as a Widmanstatten structure or isolated martensite (MA: Martensite Austenite constituent) easily occur in the bond, resulting in an even lower toughness.
In order to improve the toughness of the heat-affected zone, for example a technique that incorporates TiN in the steel by fine particle distribution to reduce coarsening of austenite grains and to create ferrite nucleation sites has been put to practical use. The bond, however, may be heated to a temperature region at which TiN dissolves. As the demand for low temperature toughness of the weld zone becomes more stringent, it becomes more difficult to obtain the above-described effect.
JP H03-053367 B2 (PTL 1) and JP S60-184663 A (PTL 2) disclose techniques in which, by dispersing fine grains in steel by means of combined addition of rare-earth elements (REM) and Ti, grain growth of austenite is suppressed, thereby improving the toughness of the weld zone.
A technique for dispersing Ti oxides, a technique for combining the capability of ferrite nucleation of BN with oxide dispersion, and a technique for adding Ca and a REM to control the morphology of sulfides so as to increase the toughness have also been proposed.
These techniques target relatively low strength steel material with a small amount of alloying elements. Unfortunately, these techniques cannot be applied to higher strength steel material with a large amount of alloying elements, since the HAZ structure does not include ferrite.
Therefore, as a technique for facilitating generation of ferrite in the heat-affected zone, JP 2003-147484 A (PTL 3) discloses a technique that mainly increases the added amount of Mn to 2% or more. With continuous casting material, however, Mn tends to segregate in the central portion of the slab. The central segregation area becomes harder not only in the base metal but also in the heat-affected zone and becomes the origin of fracture, thereby triggering a reduction in the base metal and HAZ toughness.
On the other hand, in the dual phase reheating area, carbon becomes concentrated in a region where reverse transformation to austenite occurs due to dual phase reheating, and brittle bainite structures including isolated martensite are generated during cooling, resulting in reduced toughness. Therefore, techniques have been disclosed to reduce the contents of C and Si in the steel chemical composition, inhibit the generation of isolated martensite, and improve the toughness, and to ensure the base metal strength by adding Cu (for example, JP H05-186823 A (PTL 4) and JP 2001-335884 A (PTL 5)). These techniques increase the strength by precipitating Cu by aging treatment, but since a large amount of Cu is added, the hot ductility deteriorates, inhibiting productivity.
Steel structures such as ships, marine structures, pressure vessels, and penstocks have increased in size, leading to a desire for even higher strength steel material. The steel material used in these steel structures is often thick material, for example with a plate thickness of 35 mm or more to 100 mm or less. Therefore, in order to ensure a strength such that the yield stress is at least 420 MPa grade, a steel chemical composition with a large amount of alloying elements is advantageous. In a steel chemical composition with a large amount of alloying elements, however, it is difficult to guarantee toughness of the bond and the dual phase reheating area, as described above.
With regard to this point, JP 2012-184500 A (PTL 6) proposes achieving yield stress of 420 MPa or higher and good low temperature toughness (CTOD property) even in a steel chemical composition with a large amount of alloying elements by specifying the equivalent carbon content Ceq based on a predetermined chemical composition. This proposed technique can provide a high-tensile-strength steel plate, and a process for producing the same, that has yield stress (YS) of 420 MPa or higher, which is a value suitable in steel structures for the aforementioned uses, and that has an excellent low temperature toughness (CTOD property) in the heat-affected zone of a multilayer weld formed by low to medium heat input.